Nonmetallic resistor and method of making the same



June 20, 1933.

J. A. BOYER ET AL 1,914,939

NoNMEfrALLIc REsIsTQR AND METHD oF MAKING THE ySAME:

Filed Dec. 9, 1930 n MM,

Patented June 20, 1933 UNITED STATES PATENT OFFICE JOHN A. BOYER ANI) ALMER J'. THOMPSON, OF NIAGARA FALLS, NEW YORK, ASSIGN ORS TO GLOBAR-CORPORATION, 0F NIAGARA FALLS, NEW YORK, A CORPORATION OF NEW YORK NONMETALLIORIBISTOR AND METHOD OF MAKING THE SAME Application led December This invention relates to improvements in non-metalhc resistors made from s1l1con carbide, and to improved methods in their manufacture. The invention has for its object the production of an element with improved electrical properties.

Silicon carbide resistors are well known in the art of electric heating, and possess the advantage that they may be operated at temperatures up to 14:00o C. without protection from the atmosphere. However, the electrical properties of these elements as made by the usual processes are not as satisfactory as might be desired. The resistors usually have the undesired property of greatly de creasing in resistance with rising temperature, so that as the temperature of the resistor increases, the power input at constant voltage also increases. It will be evident that, unless the operating voltage is carefully regulated, a cumulative cycle'will obtain wherein the rise in temperature will cause an increase in the power consumed by the element, and the increased power will cause an additional rise in temperature, until the resistor is heated to a temperature much beyond that which it will withstand.

The action may be very rapid when once begun, and instances have been known where the resistor has actually exploded from the -sudden increase in current, the pieces of the elementv being scattered with considerable violence.

With silicon carbide elements made by our improved process, the temperature coeflicient of electrical resistance becomes markedly positive before the maximum safe temperature of operation of the element is reached. Thus as the temperature -of the element approaches that at which rapid oxidation takes place, the current at constant voltage, instead of being increased, is considerably decreased. The strongly positive temperature coefficient affords the additional advantage that the power input is to a certain degree automatically regulated by variations in the temperature of the furnace. As an example, if the furnace is suddenly cooled by the introduction of a coldcharge, the elements will decreasein resistance and addi- 9, 1930. Serial No. 501,020.

tional current will be supplied over that which normally obtains at a somewhat higher temperature.

It is known that if silicon carbide resistors are manufactured by the process of selfebonding or recrystallization, the temperaturel coefficient of resistance is in some cases slightly positive at temperatures above TO0-800 C. However, the order of magnitude of this effect has heretofore been small and magnitude and range of positive temperature coefficient has not been subject to control. The increase in resistance with temperature becomes less as the element is used, and the coeiicient may even change in sign and become negative during the life of the resistor.

In making our resistors, we have found that there is a relationship between the ty e of grain employed and the magnitude of tlie resistance-temperature coeflicient obtained. By using a comparatively pure grain, as for example one that analyzes 99 to 99.5 percent silicon carbide, we have been able to obtain consistently, elements which show an increase lin resistance with rise in temperature of thirty to forty percent at temperatures above 70W-800O C.

As an example of a method which may be employed, in making our resistors, we prepare a mix of substantially pure silicon carbide grain, to which is added temporary bonding material such as sodium silicate. The elements are then formed into the desired shape by a suitable compacting process such as tamping. After forming, it is desirable t0 give the resistors a preliminary baking at approximately 600o C. in order to afford greater mechanical strength for handling-- bide. The green variety of silicon carbide, which varies from a dark green to a practically colorless or transparent grain, is more suitable than black silicon carbide when no subsequent purification treatments are used. vWith grain which is purified by methods which will be hereinafter described, the socalled run-of-mill grain maybe used. This grain will usually analyze about 95 to 9G percentsilicon carbide before subjection to the purification treatment.

In order to purify the grain, the crude lump silicon carbide as it comes from the furnace is crushed and is then Atreated with a suitable acid which will remove a considerable portion of the surface impurities. As an example, dilute or moderately concentrated sulphuric acid can be used. For further purification, the grain can be subjected to an additional treatment with a solution of an alkali. such as sodium hydroxide. The impurities contained in the silicon carbide are largely on the surface of the grains, and these impurities are to a. large extent removed by acid and alkali washing. A grain which is both acid and alkali washed will analyze approximately from 99 to 99.5 percent silicon carbide.

The burning bed which we prefer to use for curing our resistors, and the effect of purifying the silicon carbide grain on the electrical properties of the elements so produced, is shown in the accompanying drawing.

In the drawing:

Figure l shows a burning bed which may be used for curing the resistors;

Figure 2 shows the change in specific resistance with temperature of two representative elements made from acid and alkali washed grain, the mix containing 32.5 percent 120 mesh silicon carbide, 65 pe:'cent 280 mesh silicon carbide and 2.5 percent carbon;

Figure 8 shows the effect of acid washing the grain upon the resistance temperature characteristics of the, resulting resistor, curve C showing the Variation of the specific resistance with temperature of a resistor made from an impure untreated grain, and curve B the variation in specific resistance with temperature of a resistor made from the grain after it has been boiled with sulphurie acid to remove a considerable portion of the sur- 'face impurities; and

Figure 4 shows the effect of alkali washing subsequent to an acid washing treatment, curve E showing the variation of the specific resistance with temperature of a resistor made from grain which has been acid washed only, and curve F the variation of the specific resistance with temperature of a resistor made 'from the same grain as in curve E except that it has been subjected to an additional treatment with boiling sodium hydroxide solution.

The grain, which has been purified as above described, is classified according to grit sizes, and is made into a mix which will give the desired electrical resistance to the finished element. As an example of a mix which may be used, the following is given:

Percent 12() mesh silicon carbide, acid and alkali washed 32.5 28() mesh silicon carbide powder, acid and alkali washed (S5 Carbon (lamp black) 2.5

The resistancetemperature characteristics of typical elements made from the above mix are shown in Figure 2. rl`he minimum resistalice which the element attains between 0O and 1500O (l. is approximately twothirds the resistance at room temperature. and the in crease in resistance through the range in which the temperature coefficient is positive is from 3() to 4() per cent. The electrical resist-ance of the finished element and the resistance-temperature coefficient are both dependentI upon the manner of burning and the temperature and duration of heating during the curing process.

ln the preferred method of curing the resistor. it is desirable to mix the grain with a temporary binding agent such as sodium sili cate. and to give the formed article a preliminary baiting at approximately G00o C., before removal from the mold in order to afford additional strength for handling. The resistors are then removed from the mold, and are dipped in a slip or slurry containing fine sand. carbon and water. the sand and carbon, being in approximately the proportion of 66% parts of sand to 33t/f; parts of carbon2 The elements are then placed in a burning furnace of the type shown in Figure l, which consists of a refractoryy bottom or trough l, with electrodes at either end for conducting the current. The refractory bott-om is cov ered to a depth of about two inches with a mixture 3 of fine silica and carbon, in the proportion o't' three parts of silica to one part of carbon.

The coated resistors 4 are then placed care fully upon the bedding n'iaterial described above in end-to-end relationship extending from one electrode to the other and electrically joined to each other and to the electrode by means of graphite blocks 5 and a paste 6 comprising graphite. silicon and Na Si()- (5G-50) 0 grade 42.50 gravity. lVhen the rods have been laid down and joined to form a continuous path for the current they are covered with a silica and carbon mixture similar to that used for the bottom or bedding mix and are ready then for burning or curing. Burning or curing is then effected by applying current to the electrodes 2.

it will be seen from Figure 1 and from the above description that the resistors are so tudinal axes lie within the path taken the current 1n traversing the furnace from Ielectrode to electrode and that their axes coincide with the axis of thc path of the heating current. In fact the resistors carry most of the current after the first few minutes of the burning.

Because of the resistance of the uncured resistor, a voltage of approximately 500 volts per linear foot of furnace charge between electrodes may be required at the start to send sufficient current through the furnace to heat up the bedding material and the resistors. However, as the resistors approach the maximum temperature they aproach the normal' resistance for which they were designed and the applied voltage is cut down to avoid further substantial change. For example, a furnace load six feet long, comprising resistors designed to operate at 11.0 volts per linear foot may require aproxnnately six times 110 volts for the final applied voltage, but at the start will require approximately six times 500 volts to force the heating current through the furnace.

Resistors of different cross-sectional areas require different current valuesfor the production and maintenance of the maximum temperature and also require maintenance of the maximum temperature for somewhat different periods of time. Obviously resisters of large cross-sectional area require higher current values for the production of maximum temperatures than do smaller resistors of the sam-e general nature. It may be stated, however, by way of example, that a rod of circular cross-section approximately 1/2 inch in diameter will require approximately 4 kw. per foot for a total time of approximately 8 minutes,

The temperature of the process is believed to be between approximately 2000 and 2200o C., although exact measurement is difficult, owing to the 'fact that the material must be covered, and that more or less opaque fumes are evolved which interfere with the optical pyrometer readings. It is not necessary to regulate the temperature directly` since the temperature is usually controlled by regulating either the power input or amperage, the proper current being determined experimentally for a given diameter of element and a given amount of covering` material. It has been found that the conditions above stated` 4 kw. per ft. for 8 minutes with a 1/2 inch diameter element, will produce a satisfactory resistor.

In the method of curing described above, the crystalline particles composing the resistor are bonded by a process usually known as "recrystallization. In this process no permanent binding material is used, the sodium silicate originally present as a temr porary binder being decomposed at the temperature of curing. In the process of recrystallization, the article is heated toa temperature where vaporization of the silicon carbide takes place, and the particles grow together, presumably by evaporation and redeposition, until a strong coherent mass is obtained. In such cases it is generally pre- -sumed that the large particles grow at the expense of the small ones.

While the actual mechanism involved in obtaining a. strongly positive temperature coefiicient at the operating temperature by the use of pure grain is not completely understood, it is believed to be caused by a reduction inthe conta-'ct resistance at the surface of the various particles making up the resister.

In a self-bonded resistor made byv recrystallization there is apparently a relationship between the conductivity of the element and the magnitude of the positive temperature coefficient. Any vfactor which tends to increase the contact resistance between particles (as forl example, oxidation of the resistor during use, or the introduction of non-metallicor .non-conducting materials into the mix) either decreases the positive temperature coeflicient or causes the coefficient to become negative throughout the entire range of temperature.

The effect of the acid washing treatment is shown in Figure 3. Curve C shows the change in resistance with temperature .of a comparatively impure grain, analyzing 95.5% silicon carbide. Curve D shows the change in resistance with temperature of the same grain which has been subjected to an acid treatment, and has a purity of approximately 98.5 percent. The conductivity of the element. is increased approximately ten times, and the temperature coefiicient changes from negative throughout the entire range to positivc between 750O and 1500o C., the increase in resistance in this range with rising temperature being approximately 20 percent.

There are certain variations in the process of manufacture of a silicon carbide resistor which permit a still further control of the temperature coefficient of resistance. We have found that the grit sizes employed in the resistor mix influence to a certain degree the resistance-temperature characteristics of the resulting element. If a considerable quantity of fine powder is used, as for example to percent of a grit size between 200 and 300 mesh, and if the grain employed is substant-ially pure silicon carbide, as for example an acid and alkali washed material, an element having a very positive resistance temperature characteristic can be obtained. The reason for the effect producedl by using fine powders is not definitely known. However, we believe that the powder either offers a greater surface for contact between the intergranular particles, or evaporates and recrystallizes much more readily during the curing process than is the case with coarser crystals.

As shown by the curves and drawing, the resistors made by our improved process have a negative coeliicientof resistance up to a dark red heat., above which, however, this coefiicicnt becomes positive. A negative temperature coeiicient at low temperatures is characteristic of any silicon carbide resistor lmown at the present time. `With our iniproved elements the magnitude of the negative temperature coefficient has been considerably decreased over that which obtains with the usual silicon carbide heating element.

The presence of a strongly negative coeiiricient in the lower temperature range has always been a decided disadvantage, since it greatly decreases the rate of heating. Vith the usual silicon carbide resistor. the cold resistance is usually so high that the initial current which flows when the element is first placed in the circuit is not sutlicient to cause rapid heating. This factor has been a great disadvantage for many applications` where speed of heating is important, such as when the element is used in hot plates or stove units. As an example, in the use of al hot plate for boiling water, the total time necessary usually does not exceed fifteen minutes, and with the usual silicon carbide resistor a considerable portion of this time may be consumed in merely bringing the resistor to the operating temperature.

With our improved elements the cold resistance is approximately only one and a half times the resistance at the operating temperature. whereas with the usual silicon carbide resistor the cold resistance is from two tothree times the resistance at the operating temperature. In our resistors the initial current is increased from to 100 percent over that which obtains with the usual element having the same ampere rating at the tcmperature of operation. Such an increase in the initial current has a much greater eifect on the rapidity of heatingthan the magnitnde of he resistance indicates, since the time necc...jar v forl hcatinfT is increased several 'times for even a comparatively small decrc in the iniiaai current. rihe terni resistor her used means resistor adaptable 'for operation a. in electric heating element.

pero nt.

2. A siiicon carbide resistor comprising a. recrystallized mass of silicon carbide grains,

l which grains have been cleaned of those surface impurities which form on silicon carbide in the manufacture thereof.

3. A silicon carbide resistor having a positive temperature coeiiicient of resistance formed from the self-bonding of a mass of substantially pure silicon carbide, a large proportion of which is comminuted to powdered form, the said silicon carbide having been treated to remove the surface impurities which form on the crystal faces of the silicon carbide during its manufacture.

4. A silicon carbide resistor having a. positive temperature coefiicicnt of resistance formed from the self-bonding of a mass of substantially pure silicon carbide. a large proportion of which is con'iminuted to av mesh of 200 or finer, the said silicon carbide having been treated to remove thi` surface inipurities which form on the crystal faces of the silicon carbide during its manufacture.

5. A silicon carbide resistor having a. positive temperature coefficient of resistance formed from the self-bonding of a mass of substantially pure silicon carbide, at least 50% of which is in powdered form, the said silicon carbide having been treated to remove the surface impurities which form ou the crystal faces of the silicon carbide during its manufacture.

6. A silicon carbide resistor havingr a positive temperature coeflicicnt of resistance formed. from the self-bonding of a mass of substantially pure silicon carbide of which between 50 and 80% is in powdered form, the said silicon carbide having been treated to remove the surface impurities which form on the crystal faces of the silicon carbide during its manufacture.

7. The method of making a silicon carbide resistor having a positive temperature coefficient of resistance which comprises preparing silicon carbide in granular form, mixing therewith between 50 and 80% of powdered silicon carbide, molding a resistor from this mix and then recrystallizing the mix by passing an electric current therethrough, the silicon carbide being first cleaned to remove those mineral impurities which form on the crystals during the manufacture thereof.

8. The method of making a silicon carbide resistor which comprises reducing cominci'- cial silicon carbide to av grain size suitable. for nse in forming resistors, washing 'the 1 h'cid remove those mineral inic i. form on 'the grains during the manufacture of silicon carbide, molding the in to shape to 'form a resistor. and securing the resulting article by cting the rccrystallization thereof.

S. A recrystallized silicon carbide resistor made from silicon carbide grain which has been purified by acid treatment47 said resistor having a positive temperature coetiicicnt of electrical resistance ove at least a. part of its operating range.

l0. A recrystallized silicon carbide resistor made from silicon carbide grain which has been purified by acid and alkali washine u u l u said resistor having a positive temperature iin LIQ)

coeicient of electrical resistance over at least a part of its operating range.

11. The method of making a silicon carbide resistor having a positive temperature coefficient of electrical resistance over at least a part of its operating range, which comprises preparing commercial silicon carbide grains to a grit size for use in a resistor, treating the prepared grains to remove those surface films which form on the grains in the process of their manufacture, molding the treated grains with a binder, and recrystallizing the formed article While Passing an electric current lengthwise through the artic e. u

12. The method of making a silicon carbide resistor having a positive temperature coeficient of electrical resistance over at least a art of its operating range, which comprises the steps of preparing commercial silicon carbide grains to a grit size for use in a resistor, washing the prepared grains in an acid bath, molding the treated grains with a binder, and recrystallizing the formed article While passing an electric current length- Wise through the article.

13. The method of making a silicon carbide resistor having a positive temperature coeffcient of electrical resistance over at least a part of its operating range, which comprlses preparing commercial silicon carbide grains to a grit size suitable for use in a resistor, treating the grains by washing them in an acid and then in an alkaline bath to remove those surface films which form on the grains in the process of their manufacture, molding the treated grains with a binder, and recrystallizing the formed article while passing an electric current lengthwise through the article.

In testimony whereof 'we have hereunto set our hands. y JOHN A. BOYER.

ALMER J. THOMPSON. 

